Receivers in digital communications systems are often operated synchronously, i.e., the receivers perform operations on a received data stream in accordance with one or more clock signals. In some receivers, the one or more clock signals are adjusted in accordance with the received data stream. For example, a tracking loop, including a phase locked loop or a delay locked loop, in a respective receiver may be used to determine or extract transition information (such as 0 to 1 or 1 to 0) corresponding to data in the received data stream. The transition information may be used in a feedback loop to adjust a frequency of a local oscillator (such as a voltage controlled oscillator) thereby locking one or more clock signals to the received data stream and enabling synchronous operation of the respective receiver.
In other receivers, the one or more clock signals may be adjusted in accordance with an externally sourced reference time signal. For broadcast audio and/or video, for example, a broadcast studio may provide the externally sourced reference time signal. The externally sourced reference time signal may be associated with a data rate of an encoded data stream, which corresponds to the audio or video information. The tracking loop may determine or extract transition information in the externally sourced reference time signal. The transition information may be used to adjust the frequency of a voltage controlled oscillator (the local oscillator) thereby locking the one or more clock signals to the externally sourced reference time signal. This allows the encoded data stream to be synchronously received by the respective receiver.
There are, however, several difficulties associated with these techniques in existing broadcast receivers. Adjustable local oscillators, such as voltage controlled oscillators, and crystal oscillators are often expensive components. Such components are also difficult to integrate with other circuits (i.e., they typically cannot be included on the same integrated circuit die as other circuits). Furthermore, adjustable local oscillators are typically used in a tracking loop, which often includes a digital-to-analog converter, which also does not integrate well with other circuits and does not scale well with different fabrication processes.
In addition to these challenges, a data rate of a received encoded data stream may vary considerably during different time intervals (as is the case with MPEG video formats) and/or with different encoding formats. As a consequence, many existing broadcast receivers include two or more adjustable local oscillators, further compounding the aforementioned difficulties.
Accordingly, there is a need for lower cost broadcast receivers that can be integrated with other electronics and accommodate encoded data streams that have variable data rates.